The API and plug-in architectures are the highest priority. Another way of putting that is: never build anything in-house if you can expose a basic API and get those high-value users (the ones who are getting the most value out of the platform) to build it for you. On the Trello team, any feature that can be provided by a plug-in must be provided by a plug-in.

But whatever you do, please don’t knowingly produce undocumentation for your applications. Have some respect for your users and your fellow developers. Either take the time to write helpful documentation, or have the guts to acknowledge that there simply is no documentation.